This is one of the oddities of insurance.
I have been using Lancaster for years on my rebuilt 1986 90 (300TDi fitted) but there is no requirement for an imobiliser. A similar question on one of the other forums recently detailed that someone (with a similar agreed value policy as mine) had been told by Lancaster that they wouldn't be covered for travelling too & from a fixed place of work with their policy yet mine covers this as well.
I think that insurance is such a complicated equation that as you throw in different details (age, vehicle, address, driving history, modifications etc) it comes up with a policy that fits you only, the next person who comes along will have a different set of details so the same policy won't be suitable for them. This is why I always say that comparing insurance quotes on here is pretty much a waste of time as what fits one doesn't fit another.